Matter effects at the T2HK and T2HKK experiments


Determining the neutrino mass hierarchy and measuring the CP-violating phase δCP\delta_{CP} are two of the main aims in neutrino physics today. The upcoming T2HK (with small matter effects and high statistics) and DUNE (with large matter effects) experiments have been shown to have excellent sensitivity to δCP\delta_{CP} and the neutrino mass hierarchy, respectively. The recent T2HKK proposal aims to improve the hierarchy sensitivity of T2HK by placing one of the two tanks of the HK detector at a site in Korea, to collect data at 1100\sim 1100 km baseline. In light of the fact that DUNE will anyway collect data at 1300\sim 1300 km, we explore whether it is advantageous to collect additional long-baseline data as proposed with T2HKK, or to enhance the δCP\delta_{CP}-precision with the `conventional' T2HK by keeping both detector tanks in Japan. We do this by comparing the physics reach of these two options in conjunction with DUNE. We find that DUNE+T2HKK is better at excluding the wrong hierarchy, reaching Δχ2>175\Delta\chi^2 > 175 irrespective of the true parameters. While DUNE+T2HK can measure δCP\delta_{CP} with more precision in some parts of the parameter space, both DUNE+T2HK and DUNE+T2HKK perform equally well near the current best-fit point, giving a δCP\delta_{CP} width of around 1515^\circ. The T2HKK setup allows us to correlate and constrain the systematic errors between the two detectors collecting data from the same source, which can increase the sensitivity of the experiment by up to 25\%. Such a reduction of the systematic errors is crucial for determining the oscillation parameters with greater significance.Comment: Version accepted for publication in Phys. Rev. D (Originally titled "T2HK and T2HKK: Does more matter matter?", changed at insistence of the editorial team.

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